The present invention is related to the field of electrical measurement instruments such as meters for measuring voltage and current.
Electrical measurement instruments are known that do not require an external source of power for their operation. For example, there are portable instruments intended to be used in environments having no conventional electrical source such as a wall outlet, or where a connection between the instrument to an electrical source would be inconvenient or even dangerous. One example of such an instrument is a hand-held clamp meter that can be used to measure the voltages and currents at various points within a power distribution system. Because of the size of such a system, it is more convenient for an electrical technician to use a portable, hand-held clamp meter rather than a meter that must be connected to an external source of electrical power.
It is very common for internally-powered instruments to be powered from batteries contained within the instrument. However, battery power has drawbacks that make it unattractive in some cases. Batteries wear out and require replacing or recharging. The instrument requires regular servicing to ensure proper battery operation, and a stock of batteries or a battery charger must be maintained. Thus battery-powered instruments have maintenance costs that add to their cost of ownership. And if the instrument is not serviced regularly, the instrument may not operate reliably, likewise resulting in increased cost to the owner or user.
Apart from battery-powered instruments, there have been instruments that draw operating power from a circuit being tested or measured. For example, U.S. Pat. No. 4,422,039 issued Dec. 20, 1983 to Lee A. Davis entitled "Self-Powered Ammeter" discloses an ammeter whose AC input current is measured and also provided to a regulator that produces power for the measurement circuitry. However, previous devices generally have been tailored to very specific uses. For example, the Davis ammeter is used only to measure currents, and thus is not ideally suited for use in environments where different types of measurements must be performed.
It is desirable to be able to measure electrical signals using an internally-powered measuring device that is inexpensive and reliable, and that may be used for multiple types of commonly-taken electrical measurements.